Foamed polymeric materials are known, and typically are produced by injecting a chemical or physical blowing agent into a molten polymeric stream, mixing the blowing agent with the polymer, and extruding the mixture into the atmosphere while shaping the mixture. Exposure to atmospheric conditions causes the blowing agent to expand and to form cells in the polymer. Batch processes, that is, those in which a batch of material is treated to make a foam, rather than continuous treatment of a stream of material, are known as well.
Microcellular material is defined as cell-containing material of very small cell size. U.S. Pat. No. 4,473,665 (Martini-Vvedensky, et al.; Sep. 25, 1984) describes a process for making foamed polymer having cells less than about 100 microns in diameter. In the described technique, a material precursor is saturated with a blowing agent, the material is placed under high pressure, and the pressure is rapidly dropped to nucleate the blowing agent and to allow the formation of cells. The material then is frozen rapidly to maintain a desired distribution of microcells.
U.S. Pat. No. 5,158,986 (Cha, et al.; Oct. 27, 1992) describes formation of microcellular polymeric material using a supercritical fluid as a blowing agent. In a batch process of Cha, et al., a plastic article is submerged in supercritical fluid for a period of time, and then quickly returned to ambient conditions. In a continuous process, a polymeric sheet is extruded, then run through rollers in a container of supercritical fluid at high pressure, and then exposed quickly to ambient conditions. In another continuous process, a supercritical fluid-saturated molten polymeric stream is established. The stream is rapidly heated, and the resulting thermodynamic instability (solubility change) creates sites of nucleation, while the system is maintained under pressure preventing significant growth of cells. The material then is injected into a mold cavity where pressure is reduced and cells are allowed to grow.
Lamination of one layer or film of material to another layer or film of material to form a multi-layered structure is useful for a variety of purposes, including improved structural and mechanical properties of material, reduction of material having a desired bulk property with different, desired surface properties, and the like. Laminate articles including at least one foam layer are known, and one example of such a laminate article is artboard, which typically is made of a foamed core polymeric sheet with facing sheet adhered to one or both sides of the foam core sheet via an adhesive. For example, U.S. Pat. No. 4,180,427 (Bertsch) describe lamination of a coating on one or both surfaces of a thermoplastic sheet material, such as polystyrene foam. A corona discharge is directed against the surface of the thermoplastic material prior to lamination, which improves adhesion. U.S. Pat. No. 5,437,924 (Decker, et al.) describes a biodegradable foam core board for use as artboard, signage, display boards, etc. Decker, et al. refer to known, laminated structures including a paperboard facing adhered to a polystyrene foam core.
Decker, et al. disclose foaming a polymer foam core sheet with blowing agents such as alcohols including methanol, ethanol, propanol, and butanol or endothermic blowing agents such as those available from the Reedy International Corp., including sodium salts of carbonic, polycarbonic, polycarboxylic acids and carbonate compounds. Decker, et al. discloses using a latex binder adhesive such as one sold under the designation AIRFLEX.TM. 4000 Latex Binder from Air Products Co. for adhering paperboard laminate to a foam core sheet with pressure applied to the resultant sandwich structure until the adhesive bonding was set. Another exemplified adhesive was a self-catalyzing polyvinyl acetate water-based adhesive. Conventional lamination processes involve passing a core sheet and surface-coating laminate layers through nip rollers while heating the material to cause an adhesive to be activated and to adhere the surface layer to the core. In many cases, when hydrocarbon or other flammable blowing agents are used to foam the core, the core is allowed to degas for approximately 36 hours or more so that fewer flammable gases are present during the hot-nip roller lamination procedure, and a fire hazard thereby is avoided. Foam core sheet also may be degassed for a period of time so that the cells within the sheet do not contain high concentrations of highly-expandable blowing agents, minimizing expansion during the hot lamination process. Where relatively high molecular weight blowing agents are used, diffusion from the foam and equilibrium with the ambient environment may be relatively slow (compared to low molecular weight blowing agents CO.sub.2 and N.sub.2), thus there is motivation for allowing the material to degas for an extended period of time.
As in almost any manufacturing process, simplicity, efficiency, avoidance of unnecessary process steps, and accompanying minimized expense is desirable. Accordingly, it is an object of the invention to provide a simplified lamination process, and related laminate articles.